drm/dp: DRM DP helper/macros to get DP sink DSC parameters

This patch adds inline functions and helpers for obtaining
DP sink's supported DSC parameters like DSC sink support,
eDP compressed BPP supported, maximum slice count supported
by the sink devices, DSC line buffer bit depth supported on DP sink,
DSC sink maximum color depth by parsing corresponding DPCD registers.

v4:
* Add helper to give line buf bit depth (Manasi)
* Correct the bit masking in color depth helper (manasi)
v3:
* Use SLICE_CAP_2 for DP (Anusha)
v2:
* Add DSC sink support macro (Jani N)

Cc: Gaurav K Singh <gaurav.k.singh@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Reviewed-by: Gaurav K Singh <gaurav.k.singh@intel.com>
Acked-by: Sean Paul <seanpaul@chromium.org> (For merging through
drm-intel)
Link: https://patchwork.freedesktop.org/patch/msgid/20181031001923.31442-4-manasi.d.navare@intel.com